Madrid is bidding for a third consecutive time, while Tokyo, Doha and Baku are making their second successive attempts. It is Istanbul's fifth overall bid.

Doha is proposing to hold the games in September and October, outside the traditional July-August dates, to avoid the blistering summer temperatures in the Gulf country.

National Olympic committees had until Friday to notify the IOC of the names of any applicant cities.

The IOC will select the host city by secret ballot on Sept. 7, 2013, in Buenos Aires.

The six candidates must first submit detailed application files and letters of guarantee to the IOC by Feb. 15. The IOC executive board will meet in May to decide whether to cut any of the cities and approve a list of finalists.

Final bid dossiers will be submitted in January 2013, followed by visits to the cities by an IOC evaluation commission from February to April.

The IOC panel will issue a report assessing the technical merits of the bids at least one month before the vote. The cities will also make technical presentations to the IOC ahead of the meeting in Buenos Aires.

"Clearly the bids that begin with the most strength are Madrid, Rome and Tokyo, cities that are consolidated as three great urban centers," said Spanish IOC member Juan Antonio Samaranch Jr., son of the former IOC president and a key figure in the Spanish capital's candidacy.

British bookmaker William Hill installed Tokyo as the 2-1 favorite, followed by Rome and Madrid at 9-4, Istanbul 6-1, and Doha and Baku 14-1.

Rome, which hosted the 1960 Olympics, was the first of the cities to announce its bid months ago. The Italian capital lost to Athens in the race for the 2004 Olympics and hopes to stage the games on the 60th anniversary of the '60 Games.

"Rome is a great sporting capital and memories are still alive of the 1960 Games which ... from an economic point of view signaled the start of Italy's economic boom," Renata Polverini, the president of Rome's Lazio region, said in a statement.

Madrid, which has never held the Olympics, mounted unsuccessful attempts for the 2012 and 2016 Games. The capital finished third in the 2012 voting and second for 2016.

"Madrid has a lot of experience from its two previous candidatures," said Spanish Olympic Committee president Alejandro Blanco. "Its project has already been tested and it's just about impossible to improve on it. Now we must develop a campaign to reveal all its assets."

Tokyo, host of the 1964 Games, finished third in the vote for the 2016 Games, which went to Rio de Janeiro. The Japanese capital wants the 2020 bid to be a symbol of the nation's recovery from the March 11 earthquake and tsunami that left almost 23,000 people dead or missing.

"I would like to thank members of the IOC for encouraging us to move forward for the reconstruction from the earthquake and tsunami damage earlier this year," Japanese Olympic Committee chief Tsunekazu Takeda said in a statement.

Istanbul is back again after failed bids for the Olympics of 2000, 2004, 2008 and 2012.

"Every candidate city aspires to prepare in the best possible way and I can say that Istanbul and Turkey are going to prepare in the best way possible to organize the 2020 Games," Turkish IOC member Ugur Erdener said. "We have a good chance for hosting the 2020 Olympics."

Doha and Baku both failed to make the shortlist of finalists in the 2016 bidding.

The IOC executive board last week agreed to the Qatari city's request for a Sept. 20-Oct. 20 time frame to avoid the summer heat, when temperatures can exceed 40 degrees Celsius (104 F).

Qatar has already won the right to host the first football World Cup in the Middle East in 2022. The event will be held in June, and the desert country has proposed air-conditioned stadiums to beat the heat.

Still considered a longshot is the bid from Baku, capital of oil-rich Azerbaijan. The Azerbaijan committee said a stadium was already being built for the Olympics on the outskirts of Baku and will be completed by next year.